Being still for a model session always amazes me as I can't stay still for 30 minutes and that includes sleeping. One thing that does happen to the model is the lack of expression. To sit for a period of time takes it right out and I find the model is far far away in thought and separate from there bodies in a way. The goal along with being a good model is to stay as still as possible. I just find we lose just a little of that spark or spirit that as an artist I hope to bring to my painting. Can't have it all.